Enter your solar array size in kilowatts, your peak sun hours and a system derate to estimate the energy the array produces each day and each month in kilowatt-hours.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Calculator
Daily production
3.00 kWh/day
Monthly production
90.0 kWh/month
Formula
Daily energy = array kW × peak sun hours × derate. Monthly energy = daily energy × 30. The derate accounts for real-world losses from heat, wiring, soiling and conversion.
Worked example
A 0.8 kW array at 5 peak sun hours with a 0.75 derate produces 0.8 × 5 × 0.75 = 3.0 kWh per day, or about 90 kWh per month. Without the derate the ideal figure would be 4.0 kWh per day, which real systems do not reach.
Panels are rated under standard test conditions, a controlled 25 degrees Celsius cell temperature and 1,000 watts per square meter of light. In the field, panels run hotter, which lowers output; wiring and the inverter or controller lose a few percent; dust, shading and aging cost more; and energy stored in a battery loses some on the round trip. A derate factor of about 0.75 captures these combined losses, which is why a 0.8 kW array yields around 3.0 kWh on a 5-hour day rather than the ideal 4.0 kWh.
How accurate is the 30-day month estimate?
Multiplying daily output by 30 gives a useful average, but real months vary. Peak sun hours change with the season, so summer months can exceed the estimate and winter months fall well below it. The figure also assumes typical weather; a run of storms will pull a month down. Treat the monthly number as a planning average, and for year-round off-grid reliability size against your worst month rather than the annual mean.
What array size do I enter?
Enter the total nameplate wattage of your panels, converted to kilowatts by dividing by 1,000. Two 400 W panels are 800 W, or 0.8 kW. Use the panels actual rated wattage, not what you hope to harvest; the derate factor already reduces the rating to a realistic output. If you are planning rather than measuring an existing array, the solar panel size calculator works backward from your energy needs to the array you require.
How do peak sun hours change output?
Output scales directly with peak sun hours: double the sun hours and you double the daily energy for the same array. A 0.8 kW array making 3.0 kWh per day at 5 sun hours would make about 2.4 kWh at 4 sun hours and 3.6 kWh at 6. Peak sun hours depend on location and season, so a sunny southwestern site produces far more from the same panels than a cloudy northern one. The linked peak-sun-hours table lists typical values by state.
Can I use this for a grid-tied system?
Yes, the daily and monthly energy estimate works for any array, grid-tied or off-grid. Grid-tied systems without batteries skip the battery round-trip loss, so they sometimes justify a slightly higher derate, around 0.8, which raises the estimate a little. The calculation of energy as array kilowatts times sun hours times derate is the same. For billing or net-metering planning, compare the monthly figure against your utility statements over a full year.
Source: Production estimate: daily kWh = array kW × peak sun hours × derate; monthly = × 30. Peak sun hours from NREL averages. · All sources